This invention relates to a multilayer foam glass block having a dense glass surface layer and method of producing the same. The multilayer foam glass block is suitable for use as an interior or exterior decorative material in buildings or as an exterior wall material.
Foam glass, i.e. foamed glass, is low in specific gravity and high in heating-insulating ability but is relatively low in mechanical strength and also in resistance to flawing of surfaces. It is known, as shown in JP-A 50-123108 and JP 60-12937 for instance, to fusingly integrate a foam ceramic or glass layer and a dense and foamless glass layer into a block of shielded foam glass. As the foam glass layer is covered with the dense glass layer the obtained block is fairly high in rigidity and mechanical strength and has an outer surface high in hardness and resistant to flawing. Besides, the dense glass surface layer affords the foam glass block with glossiness and graveness characteristic of glass and produces a valuable ornamental effect.
However, problems are involved in tightly covering a low-density foam glass layer with a dense glass layer. Usually foam glass has an apparent specific gravity (hereinafter the term "bulk density" will be used in this sense) below 0.8 whereas the bulk density of the dense glass layer practically agrees with the true specific density of the employed glass, which is about 2.5. Becuase of such a great difference in density there arise significant differences in thermal characteristics between the foam glass layer and the dense glass layer. Particularly the dense glass layer is far higher in heat conductivity than the foam glass layer. Therefore, at the stage of annealing a foam glass block which is produced by a usual firing process and includes a unitarily formed dense glass layer it is inevitable that the dense glass layer solidifies, with some shrinkage, considerably faster than the under-lying foam glass layer no matter how precisely the cooling condition may be controlled. As a natural consequence, strains are induced between the two glass layers so that adhesion at the interface becomes insuf-ficient or defective. Accordingly the obtained foam glass block is liable to undergo cracking from the interfacial region by external and even relatively light shocks or by fatigue attributed to repeated weathering.
JP-A 59-111948 shows providing a glazed surface to a foam glass block by applying a cementing material containing a large amount of a hydraulic material to a simple foam glass block to fill a portion of open-pores in the foam glass block with the applied material and also to form an intermediate layer on the surface of the foam glass black, then applying a glaze onto the intermediate layer and firing the whole block to thereby finish a hard and dense surface layer of glaze. However, even in this case a considerable difference in heat conductivity between the foam glass layer and the foamless intermediate layer raises problems. Straining between these two layers is further promoted by significant thermal expansion of the hydraulic material such as cement or gypsum used in the intermediate layer. Although an anchoring effect of the hydraulic filling is expected, adhesion between the foam glass layer and the overlaid dense layer is not fundamentally improved and there is no merit in respect of productivity and production cost.
According to JP 49-28251, impact resistance of a foam glass block having a dense surface layer formed of vitreous enamel becomes high when the foam glass overlaid with the enamel layer has a bulk density higher than 0.4. However, the problem of insufficent adhesion between the two layers still remains unsolved, and it is not rarely that foam glass is desired to have a bulk density below 0.4.